FACTORS OF ENVIRONMENT. 465 



Some thrive exposed to strong sunlight, others in the shade, and 

 so on. Not only is the individual plant influenced by these con- 

 ditions of environment, but colonies or societies of plants, as 

 they exist in a natural state, which we call their home, are 

 likewise influenced. In fact these conditions of environment 

 largely determine the household relations of plants, i.e., the eco- 

 logical relations, the study of which we call ecology, a study of 

 plants in their home, or natural environment. These condi- 

 tions are known as ecological factors. The ecological factors 

 are in general of three sorts: ist, physical factors; 2d, climatic 

 factors; 3d, biotic factors. Some of the physical factors are 

 water, heat, light, wind, chemical condition of the soil, physical 

 or mechanical condition of the soil, etc. The climatic factors 

 are rainfall and general atmospheric humidity, the broader 

 temperature limits, great changes in temperature for long 

 periods. The biotic factors are certain animals, the plants 

 themselves, and even man. 



I. Physical Factors. 



901. Water. Importance of water. Water is regarded as 

 one of the most important of the ecological factors, though it 

 should be borne in mind that no factor is operative unless cer- 

 tain other factors are also favorable. The plant responds only 

 to the favorable action of several of the factors, and it is exceed- 

 ingly difficult to ascribe a definite value to any one of them. 

 We can study the influence of one by maintaining a certain 

 degree of constancy of all the others. That water is a very im- 

 portant factor, however, can be seen from the fact that most 

 plants contain a large percentage of water, which in the case of 

 land plants is rapidly lost by transpiration, and must be replaced 

 by absorption of more water from the soil, while in the case of 

 water plants the effect of removing them for even a short while 

 from the water and exposing them to dry air is easily seen. 



902. A modicum of soil-water. For all species there is a 



